1. Field of the Invention
The present invention relates to microwave radiometry and more particularly, to a wideband synthetic-aperture multichannel correlation receiver.
2. Description of the Prior Art
There are certain phenomena on our Earth which have great significance, from military and/or commercial points of view. One such phenomenon is the water temperature of the oceans or other large bodies of water. Knowing the temperature of the water over large areas is of importance for military purposes. It is also of importance from a commercial point of view, since ocean water temperature affects commercial fishing. Theoretically, one can take direct measurements of the water temperature at specific locations. However, due to the large surface area of the oceans, taking direct water temperature measurements over any large surface area of hundreds of thousands of square miles is clearly impractical.
At present, there are several buoys which are permanently anchored at very precisely-known locations in the Atlantic Ocean off the Eastern Coast of the United States. On each of these buoys a very precise thermometer is used to measure water temperature. Equipment is also included to enable a satellite, which makes repeated passes over the area, to receive the actually measured water temperature at each buoy as the satellite's antenna points at the buoy. Thus, values of precisely known water temperatures at the buoys are available in the satellite for transmission to Earth. However, the number of buoys is very small.
At present, there are only about 6 such buoys in locations from off New England down to off South Carolina and out several hundred miles, thus being spread out over an ocean surface area of over 150,000 square miles. Thus, even though the water temperature at each buoy is known, the water temperature at small areas, hereafter referred to as pixels, at significant distances from any of these buoys is not known.
One may obtain a very rough approximation of the temperature of a pixel between two buoys based on the measured temperatures at the buoys. However, these approximations are too rough and, therefore, are of little, if any, use. A need therefore exists for an arrangement for obtaining reasonably precise measurements of sea or water temperature over a large area.
The above-referrred to buoys also provide the satellite with measurements of other phenomena, e.g. wind temperature, wind speed. An arrangement which can use these phenomena measurements to obtain precise measurements thereof at locations remote from any of the buoys is highly desirable.
Some prior art systems use an arrangement wherein signals are received by an antenna and fed to a plurality of radiometers, each tuned to a separate frequency. Measurements are made based on a theoretical model in which it is assumed that signals at different frequencies are indicative of different phenomena. For example, signals at frequencies of 6 GHz, 18 GHz, 21 GHz and 27 GHz are assumed to be indicative of water temperature, moderate or heavy rain, water vapor, and light rain, respectively. In the prior art any calibration is not done against the true surface, i.e. the water itself. Rather, periodically the antenna is disconnected from the radiometers and signals are applied to them from special sources of hot and cold temperatures.
Such a prior art system does not provide sufficiently accurate data, due to the fact that any calibration is not done against the true surface. All measurements are subject to error due to the conditions of the atmosphere between the ocean surface and the antenna. Also, the outputs of the radiometers are not combined in a manner so as to improve signal to noise ratio, by greatly increasing the resolution to be greater than even the aperture of the narrowest beam.